【作者】 倪冰；

【导师】 邹宗树；

【作者基本信息】 东北大学 ， 钢铁冶金， 2009， 博士

【摘要】 板坯结晶器内钢液吹氩过程对连铸生产和铸坯质量都有深刻的影响。在实现防止水口堵塞和避免钢液二次氧化的同时,吹氩引起的流型转换是困扰连铸生产的一个难题。气泡在双循环流流场内实现冶金功能如促进夹杂物上浮的同时,也会有一部分在凝固坯壳上被捕获形成缺陷,如何扬长避短也一直是一个亟待解决的问题。因此,揭示和把握结晶器内钢液—氩气—夹杂物三相运动行为特征和气泡被凝固坯壳捕获的过程以及气泡去除夹杂物的机理显得尤为重要。本文以常规板坯连铸结晶器吹氩过程为研究对象,建立物理模型及其相应的数学模型,用物理实验结果验证数学模型。二者相吻合后,建立实际工艺条件下的数学模型。对吹氩对流场的影响、气泡在凝固坯壳上被捕获和促进夹杂物上浮等几个方面进行了模拟研究和理论分析,以实现进一步提高铸坯质量的目的。针对引起铸坯质量波动的结晶器内流型转换问题,本文建立结晶器物理模型研究体系,利用示踪剂显示吹气量对流体流型的影响,通过调整气量来确定一定工艺条件下的临界吹气量,找出单双循环流转变的条件。利用连续性方程、动量方程、湍流模型和离散相模型模型建立了描述结晶器内气液两相运动的数学模型,经过物理实验验证后,将数学模型用于描述钢液—氩气体系的行为特征。提出一个包含气流量和水流量、气体和液体密度、板坯尺寸和水口出口角度参数在内的无量纲数[(Jgρg+Jlρl)(Jg+Jl)cos2θ]/ρlgD,利用该数和气液体积比Qg/Ql,的关系作为形成单双循环流的判定条件,在曲线以上区域为单循环流,以下为双循环流。判定条件普遍适用于板坯连铸结晶器。围绕双循环流流场内的气泡运动行为,在物理实验中利用高速摄像系统记录气泡的运动过程,用专业图像处理软件分析气泡运动图像,总结速度、轨迹和含气率等参数来描述其运动行为。水口出口面积和倾角一定时,水流量是影响气泡分布状态的决定性因素,其他因素影响很小。利用物理实验所得到的气泡运动终速和直径的关系作为初始条件,代入准单相数学模型求出水模内的含气率。和水模结果对比后证实了准单相模型的有效性,可以用来反映钢液的含气率分布。气体不仅改变液体速度的大小还改变了速度的方向,对流场的影响体现在宽面、自由液面及窄面上。本文采用自由能的观点,对在液体中球形气泡和固体壁组成的三相体系中,气泡的附着及其逆过程(脱离)进行了热力学分析。在考虑浮力和表面张力的情况下,在液体内的单个球形气泡附着在固壁上这一物理过程中,无论在固壁上形成的气泡是球缺还是球冠,体系的自由能都会减少,气泡的附着过程自发进行。必须有额外做功才能使气泡发生。用高速摄像仪记录了水模中三种金属丝布置方式下的气泡被捕获的过程,来模拟凝固枝晶捕获气泡现象。气泡被捕获的过程包括气泡靠近并与金属丝发生碰撞、形成液膜、形成新的气固接触界面和稳定附着等几个过程。最后,针对夹杂物依靠自身浮力上浮和被气泡捕获两种去除方式,进行物理实验来研究夹杂模拟物的去除效果,定量确定水流量、透气砖类型和气量对夹杂模拟物去除效率的影响。随着水流量的提高,各种粒径的夹杂模拟物粒子的去除率均呈先迅速下‘降后缓慢增加的趋势。使用弥散透气砖比多孔直通型透气水口的夹杂模拟物去除率分别有所提高。在同一水流量下,随着气量的增加,去除率增加较为平缓。用高速摄像系统准确记录气泡去除夹杂模拟物的运动过程。气泡粘附是去除夹杂物的主要原因,单个气泡粘附去除夹杂物的过程可分为气泡与夹杂物靠近发生碰撞、液膜形成、液膜减薄形成新的气固接触界面、夹杂物在气泡表面上滑移至底部、夹杂物与气泡动态稳定粘附和一起上浮等5个过程。夹杂物在气泡表面上滑移时处于不稳定状态,只要有轻微的扰动,夹杂物就会失稳脱离液膜。在准单相数学模型之上加入离散相模型,建立的钢液—氩气—夹杂物体系数学模型可以描述夹杂物去除情况,夹杂物的去除率变化规律与物理实验相类似。更多还原

【Abstract】 Argon gas injection on fluid flow in continuous slab casting mold plays an important role in the casting process and the slab quality. While it can prevent clogging of the SEN (Submerged Entry Nozzle) and secondary oxidation of molten steel, the conversion of flow pattern of molten steel which is changed from double recirculation to single recirculation is still a technical problem. The bubbles in the double recirculation can promote the inclusion to be removed, meanwhile, some of them are entrapped on the solidified shell to form internal defects in steel products, and how to strength the beneficial function of bubbles and circumvent their weaknesses has been a serious problem. Therefore, it is highly emphasized to reveal the behavior character of steel-gas-inclusion three-phases and bubble attachment on the solidified shell, as well as interaction of inclusion removal by bubble. They are three core questions conducted to understand and optimize the metallurgical factors on slab quality and productivity in this paper. The gas injection on fluid flow in a continuous slab casting mold was analyzed by means of water model simulation and numerical simulation. The mathematical model under the condition of produce process was built after two simulation methods matched.After the establishment of physical model for the mold according to similarity principle, the flow pattern with different air flowrates have been observed by using red color matter to trace and the critical air flowrate have been achieved under the different casting conditions in the mold model. Meanwhile, the mathematical model to simulate the gas-liquid two-phase flow in the water model has been developed by conservation equations for mass continuity and momentum, turbulence model and DPM (Discrate Phase Model). Since the results of physical model are greatly essential to validate and perfect the numerical model, the mathematical model can describe the behavior chariactor of molten steel-argon two-phase flow, so the formed conditions of a single or double recirculation flow pattern in the slab continuous casting mold with argon gas injection are quantitive described. As a result, the functional relationship of dimensionless criterion number [(Jgρg+Jlρl)(Jg+Jl)cos2θ]/ρlgD and gas-liquid volume ratio Qg/Ql, which the number is integrated with the relationships including flowrate of gas and liquid, density of gas and liquid, slab size, and tilt angle of the nozzle port, is proposed to discriminate the boundary of single and double recirculation flow pattern. The criterion condition, the area of single recirculation flow pattern is above the critical curve and that of double recirculation flow pattern is below the curve, is adapted to the conventional mold for slab caster.The bubble distribution and motion including velocity, pathlength and gas fraction parameters in the water model were invested by using the high-speed video camera measurement and the image processing. The results show that the bubble distribution is dominated by water flowrate as the nozzle outlet area and its angle is fixed, which is hardly influenced by other factors such as gas flowrate, SEN shape, and surbumerged depth, so the bubble distribution can be regulated by controlling the casting speed in the mold. The gas fraction in water model can be calculated by using HFM(Homogeneous Flow Model) whose initial condition is the relationship between the bubble terminal velocity and diameter measured in the physical experiment, so can the gas fraction in the mold. The bubbles weaken turbulence fluctuation of water flow, which can be observed from the variation of the flow velocity direction and the velocity value at the wide face, the free surface and the narrow face.When the spherical bubble attaches to solid wall in liquid, the free energy change formula in system is deduced to prove that the bubble attachment process at the balance between the buoyancy and the surface tension on the bubble can occur spontaneously with surface area reduced. It is because of this thatΔG<0 in system at the condition of the contact angle ranging from 0°to 180°. Meanwhile, the phenomenon can not happen becauseΔG=0 in system when the contact angle is equal to 0°or 180°. The attached bubble can not break off from the wall unless the extra power is applied. Moreover, the bubble attachment to metal wires in the water model was recorded by the high-speed video camera, which the process could be described as four steps including the bubble approaching to and attachment to the metal wires, forming liquid film between the bubble and the wires, forming new gas-liquid interface, the bubble sticking to the wires stably.Finally, the removal efficiency of inclusion by bauycancy and attachment to bubbles was studied quantitatively at five different water flowrates, by using two different purging plug types in SEN, and at four different gas flowrates. At first, the removal efficiency decreases rapidly as the water flowrate increases to a critical value, while it intends to slow as the water flowrate continues to enlarge. Secondly, the removal efficiency by using porous brick is increased than that by using multihole plug. Thirdly, as the gas flowrate enlarges, there is a modest increase of the removal efficiency due to three reasons including the probability of an inclusion attachment to a bubble, the bubbles distribution and their quantities. Moreover, the inclusion attachment to the bubble, which was the main method of inclusion removal, was recorded and vertified by the high-speed vedio camera. The process of an inclusion attachment to a bubble can be described as five steps including the bubble approaching and attachment to the inclusion, forming liquid film between the bubble and the inclusion, forming new gas-liquid interface after liquid film becoming thin, the inclusion slipping from the bubble top surface to bottom, the inclusion oscillation at the bottom of the bubble and float with the bubble. The inclusion can leave off from liquid film if there is any disturbution when the inclusion slips on the bubble surface. The removal efficiency of inclusion in the molten steel can be simulated by using DPM and HFM, and present the similar rule with the physical exiperiment.更多还原